1. Field of the Invention
The present invention relates to a circuit board, and more particularly to a circuit board that is bent when used.
2. Description of the Related Art
As a conventional circuit board, for example, a printed circuit board disclosed in Japanese Patent Laid-Open Publication No. 2002-305382 is known. FIG. 38 is a sectional view of the printed circuit board 500 disclosed in Japanese Patent Laid-Open Publication No. 2002-305382. FIG. 39 is a sectional view of the printed circuit board 500 in a bent state.
The printed circuit board 500, as illustrated in FIG. 38, includes a four-layer portion 500a, a heat radiating portion 500b, and a connecting portion 500c. The four-layer portion 500a, the heat radiating portion 500b and the connecting portion 500c are preferably formed by laminating insulating base plates 523a through 523f. More specifically, the insulating base plate 523c extends in a right-left direction. In the left-side portion of the insulating base plate 523c, the insulating base plates 523a and 523b are laminated on the upper main surface of the insulating base plate 523c, and the insulating base plate 523d is placed on the lower main surface of the insulating base plate 523c. Thereby, the four-layer portion 500a is formed. Also, in the right-side portion of the insulating base plate 523c, the insulating base plate 523e is placed on the upper main surface of the insulating base plate 523c, and the insulating base plate 523f is placed on the lower main surface of the insulating base plate 523c. Thereby, the heat radiating portion 500b is formed. The connecting portion 500c is a portion between the four-layer portion 500a and the heat radiating portion 500b. 
In the printed circuit board 500, the thickness of the four-layer portion 500a and the thickness of the heat radiating portion 500b are greater than the thickness of the connecting portion 500c. Accordingly, the connecting portion 500c is more flexible than the four-layer portion 500a and the heat radiating portion 500b. Therefore, when the printed circuit board 500 is used, the connecting portion 500c is bent.
In the printed circuit board 500 disclosed in Japanese Patent Laid-Open Publication No. 2002-305382, however, the connecting portion 500c is likely to break at a portion near the border with the four-layer portion 500a as illustrated in FIG. 39.
More specifically, when the connecting portion 500c is bent, a tensile stress acts on and around the outer main surface of the bent connecting portion 500c. Accordingly, the portion of the connecting portion 500c around the outer main surface is stretched. As illustrated in FIGS. 38 and 39, in the four-layer portion 500a, the insulating base plates 523a and 523b are laminated on the upper main surface of the insulating base plate 523c, and the insulating base plate 523d is placed on the lower main surface of the insulating base plate 523c. Therefore, the insulating base plate 523c in the four-layer portion 500a is less stretchable than the insulating base plate 523c in the connecting portion 500c. In this structure, when the connecting portion 500c is bent, the insulating base plate 523c in the connecting portion 500c is stretched excessively so as to compensate for the short of stretch of the insulating base plate 523c in the four-layer portion 500a. Thus, there is a risk of breakage of the connecting portion 500c at a portion near the border with the four-layer portion 500a. 